1. Field of the Invention
The invention relates to a method of operating a liquid-liquid heat exchanger which has a plurality of upwardly directed tubes for upward movement of a first heat exchanging medium while a granular mass is kept fluidised in the tubes by the first medium and, around the tubes, a chamber for downward passage of the second heat exchanging medium.
A liquid-liquid heat exchanger of this type is disclosed in Dutch laid open patent application No. 7703939 (GB No. 1,592,232), which explains how the apparatus is dimensioned so that a condition can be created, during operation, in which the movement and/or conveyance of the granular mass in each of the tubes is almost identical.
By means of a fluidised granular mass in the tubes, more efficient heat transfer to the inner walls of the tubes is achieved, thereby reducing the costs of construction and operation of the heat exchanger, compared with a heat exchanger of the same capacity without a fluidised granular mass. This applies particularly if a liquid which has a highly contaminating action on the tube wall flows through the tubes, because the fluidised granular mass exerts a slightly abrasive action on the tube wall, thereby limiting contamination and in many cases even eliminating it.
Practical tests have shown that a heat exchanger which is provided with a fluidised granular mass in the tubes can have a heat transmission coefficient (K value) five times higher than a conventional heat exchanger which does not make use of a fluidised granular mass. It has also been shown that in many cases heat exchangers with fludised particles in the tubes can still be used in situations where conventional heat exchangers can no longer generally be used. For example, unless a heat exchanger can be used, a process liquid can only be heated by direct steam injection, with all the unfavourable consequences of this, such as loss of condensate and dilution of any process flow.
Thus, it may be stated that a heat exchanger with a fluidised granular mass in the tubes performs superior heat transfer, even at low or very low speeds of the first heat exchanging medium, and that serious contamination of tube walls can be overcome very effectively with it.
The extremely good heat transfer at low speeds (flow rates) of the first heat exchanging liquid may lead a designer to use a short length for the tubes and to use a large number of parallel tubes. In a number of cases this may be favourable, but sometimes this low flow rate can be unfavourable because of the large numbers of tubes involves large tube plate diameters and a great amount of drilling work. The low flow rate frequently also means that a large cross-section of flow is provided for the second heat exchanging liquid on the outside of the tubes. This means that the second heat exchanging liquid can only flow at a slow rate along the outside of the tubes, as a result of which the heat transfer to this outer side of the tubes is reduced, with unfavourable effects on the heat transmission coefficient of the heat exchanger.
The flow rate of the second heat exchanging medium may be increased, for example, by using a large number of baffles outside the tubes, but this in turn again increases the cost price of the heat exchanger considerably, and is therefore undesirable.
A heat exchanger with a fluidised granular mass in the tubes, is also described in Dutch patent application No. 8102024 (EP No. 822004370), both published after the priority date here claimed. In this case, however, the above-mentioned disadvantage of low flow rate of the second medium is avoided by using a falling liquid film of the second medium on the outside of the tubes. This results in very good heat transfer, despite a low total mass flow of the second medium. However, one disadvantage of this is that in many cases a separate pump is required to discharge the second medium. There is also the risk that gases may dissolve from the volume outside the tubes into the second medium as it flows along the tubes in the form of a film. Such dissolved gases are often undesirable if the second medium has to be re-used in a particular process, for example, if boiler feedwater is the second medium.